In an electrophotographic apparatus and an electrostatic recording apparatus, a toner is adhered to an electrostatic latent image formed on a photoconductor, which is transferred to a recording medium and fixed on the recording medium by heat, and a toner image is formed.
Also, in general, a full-color image is formed by reproducing colors using toners of four colors, namely yellow, magenta, cyan and black. A toner image that the respective toners are superimposed on a recording medium is heated and fixed, and a full-color image is formed.
Generally speaking, a toner used for developing an electrostatic image includes colored particles including a colorant, a charge controlling agent and other additives in a binder resin, and as a manufacturing method thereof, there are roughly a pulverization method and a polymerization method.
In the pulverization method, a toner composition is prepared by melt-mixing and dispersing uniformly a colorant, a charge controlling agent and an offset preventing agent in a thermoplastic resin. The toner composition is pulverized and classified, and a toner is manufactured.
According to this pulverization method, the toner may be manufactured at low cost. However, it is likely that the toner has a broad particle size distribution, and there is a disadvantage of a very low yield due to classification.
Also, in the pulverization method, it is difficult to disperse the colorant and the charge controlling agent uniformly in the thermoplastic resin. Non-uniform dispersion of the ingredients adversely affects toner fluidity, developability, durability and image quality.
In recent years, higher image quality is further desired to satisfy high resolution and high definition close to photography or printing, and as a method for manufacturing a toner having a small particle diameter and a narrow particle size distribution, toner particles of an irregular shape are obtained by associating resin particles by an emulsion polymerization method.
Reduction of a particle size of the toner largely accounts for improved number of image output per unit mass of toner due to reduced amount of toner adhesion on paper, etc. per unit area (low M/A).
As a means to reduce an adhesion amount of a toner, a technique to create voids inside toner particles has been tried.
A method to achieve a reduced adhesion amount of a toner (low M/A) while maintaining the toner to a minimal particle size to ensure developability, transferability and fixability is disclosed (see PTL 1). However, it does not describe details of a method to control an amount of the voids.
A method to control internal voids of a toner by controlling a solvent volatilization rate in the inside of particles is disclosed (see PTL 2). However, the voids formed by the solvent volatilization are not only spherical but also crack-shaped. When the particles have a large void fraction, the particles collapse due to lack of mechanical strength, which causes problems such as carrier spent.
In a method for manufacturing a polymerization toner, it is also important to achieve high production rate with high efficiency as well as quality. It is required to improve the number of image output per unit mass of toner by controlling the void fraction inside the toner particles to maintain the mechanical strength but without degrading the particle size distribution by addition of additives.